The invention relates to a heat transfer medium, in particular for use in solar thermal power plants, and a method for operating a solar thermal power plant.
Energy production by solar thermal power plants, which are based on the technologies of parabolic trough and Fresnel reflectors, heliostats and solar towers, is becoming increasingly important.
Solar thermal power plant concepts normally use solar power, in order—by beam concentration and bundling mechanisms, such as for example a multitude of cascaded mirror geometries—to heat up a heat transfer fluid (HTF) or heat transfer medium within an expanded absorber pipeline circuit which then converts liquid water from a reservoir into high-pressure water vapor via a heat exchange process, in order thereby to generate electricity by a turbine. The cooled HTF runs through the heating process again by solar power and ensures continuous power generation.
The heat transfer medium used in this solar field primary circuit is subject to stringent requirements. Such a fluid should therefore in particular have a very low liquidus temperature, since rapid cooling may otherwise result in solidification within the absorber pipe in the absence of solar radiation. This risk is particularly prevalent in the hours of darkness, when such a power plant does not produce any electricity per se. This solidification may possibly be counteracted during the night by external combustion (e.g. energy withdrawal from a hot thermal reservoir, known as Thermal Energy Storage or TES, electrical heat tracing or even thermostatic control by supplying combustion heat from fossil fuels such as gas), in order to maintain the fluid phase state and thus the pumpability of the HTF. Accordingly, the higher the melting point or liquidus temperature of the heat transfer medium, the more intensive the unwanted internal energy consumption for maintaining the heat of the absorber pipe system.
This namely also reduces the effectiveness of a solar thermal power plant complex. At the same time, high maximum operating temperatures (and, similarly, high decomposition temperatures) are required, since the efficiency of a power plant is known to increase disproportionately with the temperature.
To guarantee the maximum lifetime of the solar field circuit pump system and to keep the pump power consumption as low as possible, the HTF should have high fluidity. At the same time, the HTF in the liquid state should combine high thermal conductivity with high specific thermal capacity. All of these factors are what chiefly determine the power generation costs of future solar thermal power plants, and thus the point in time at which economic cost parity is reached and the competitiveness of such plants.
There is therefore a great deal of interest in the quest for novel, non-toxic, low-melting (with melting temperatures preferably below 100° C.), thermally stable and highly fluid heat transfer media with low procurement costs in the multitonne range.
The non-eutectoid mixture of 60% by weight of NaNO3 and 40% by weight of KNO3, known as solar salt, which has a melting temperature of 240° C., is known for this purpose.
However, a relatively high melting point such as this can result in inefficiency in a solar thermal power plant for use in the high megavoltage range. Admixing, i.e. ternerization and quaternization of the established Na—K—NO3 mixture, by further cations with different ionic radii, is used to reduce the melting temperature. The ternerization of the cation quantity with Ca2+ ions in the form of calcium nitrate additions (cation base: 21 mol.-% Ca2+, 49 mol.-% K+, 30 mol.-% Na+) thus leads to a reduction in the liquidus temperature to 133° C. (cf. “Phase Diagrams for Ceramists”, E. M. Levin, C. R. Robbins, H. F. McMordie (Eds.), Volume I and II, American Ceramic Society, 1964).
U.S. Pat. No. 7,588,694 B1 discloses a heat transfer medium which is based on solar salt, into which is mixed different calcium salts to lower the melting point; a lowering of the melting point is also effected by the addition of lithium cations.
By further admixture of lithium ions in the form of LiNO3, U.S. Pat. No. 7,588,694 B1 states that melting ranges of Ca—NaK—Li—NO3 mixtures at approx. 97° C. can be achieved.
Even though the use of nitrates, which—as fertilizer waste—have been virtually unused to date, may indeed be highly economical, the use of lithium salts as an HTF medium for the purposes of commercial, industrial-scale implementation in the solir thermal power plant sector is precluded, since it is expected that, in the future, there will be increased competition for scarce global supplies of lithium-based material from the accumulator industry, which uses lithium salts for the preparation of lithium-ion batteries.